Following an impairment in the normal cooling systems of nuclear reactors leading to a loss of cooling accident ("LOCA") and following shutdown of the fission chain reaction, there is a need to provide for rejection of residual heat, principally from the decay of fission products. For conventional water-cooled reactors, residual heat is rejected using pumps powered, in the absence of normal power supplies, by diesel generators. Redundancy is achieved with multiple trains of diesel generators and pumps, but reliability is limited by a lack of diversity in such an approach.
Modern designs of nuclear reactors avoid total reliance on active emergency cooling systems. A well engineered and well maintained passive system is thought to be more reliable than an active system.
Passive systems generally rely on natural convective forces to transport heat from inside to outside containment. Passive cooling measures promote the rate of heat transfer from the containment atmosphere to the containment walls. In such designs, the containment wall is usually made of steel in order to take advantage of its high thermal conductivity. However, other methods of promoting heat transfer to the containment wall are usually required. For example, in U.S. Pat. No. 5,049,353 issued Sep. 17, 1991 to Westinghouse Electric Corp., there is disclosed a reactor design in which containment cooling is effected by heat transfer through a steel containment vessel to the outside air flowing upwards by natural convection. The natural convection is enhanced by the use of an annular baffle about the containment wall and open at the top and bottom to improve airflow over the wall. The air-side heat transfer is augmented by the evaporation of water, which flows from an elevated tank and over the outside of the containment vessel against the up-flow of air.
Such designs that rely on flow past the containment wall to remove heat from the containment atmosphere are inherently inefficient in that the heat sink, being the reactor wall, is inefficiently distributed in elevation. Heat should be transferred from the containment atmosphere at the highest possible elevation to maximize natural circulation and heat transfer from containment gases. Another drawback with conventional designs is the poor heat transfer coefficient that is usually encountered with flow tangential to a surface, as would apply both inside and outside the containment wall. A related drawback is the limited surface area for heat transfer that is available in these designs.
It has also been proposed to use baffle walls inside containment to create a downward flow of air against the containment wall to improve heat transfer. However, the effectiveness of the use of an in-containment baffle wall to promote the passive circulation of containment atmosphere depends in large part on the architecture within containment. The proliferation of equipment and partitioned spaces within containment in the annular area between the baffle wall and the containment wall tends to interfere with the natural convective circulation. While the use of baffle walls greatly enhances convective circulation and heat transport to the containment wall, engineering and design considerations usually restrict their application to discrete locations about the periphery of the containment or require that equipment and partitions be placed in the annular flow area thereby disrupting the flow.
Given the requirement that nuclear reactors are required to be exceedingly structurally sound to withstand missile attacks and seismic events, an external concrete wall is imperative. Accordingly, conventional reactor designs using steel or other suitable metal having a high thermal conductivity to form the containment wall require two walls, an inner steel containment wall to ensure high levels of heat transfer and an outer concrete shielding wall.
Some designs augment the passive removal of heat by providing water reservoirs as a heat sink. For example, the Westinghouse AP600 reactor uses a water pool inside containment to condense steam from the RPV. However, while the in-containment pool cools the RPV, it does not cool containment atmosphere. Indeed, the in-containment pool is designed to boil and the heat must ultimately be transferred by containment atmosphere to the externally cooled containment walls.
Out-of-containment pools have been proposed for containment atmosphere cooling. In U.S. Pat. No. 5,276,720 issued Jan. 4, 1994 to General Electric Company, there is described a boiling water reactor design including a passive containment atmosphere cooling system using an out-of-containment pool. A steam condenser is located in the pool. Steam from containment is applied to the tube side of the condenser and is condensed to limit the increase in pressure and maintain containment integrity. In a limiting accident, the external pool water is permitted to boil. The effectiveness of this system has certain inherent limitations. By placing the condenser inside the pool, the condenser size, and hence rate of heat transfer is limited, particularly when the water level of the pool boils down to low levels. In addition, the system is inherently limited in the volume of steam that can be delivered from containment, through lines to and from the remotely located condenser. In addition, venting of non-condensibles back to containment presents practical difficulties. Finally, the containment cooling effected by the condenser is not optimized to promote the natural convection of atmosphere within containment and therefore does not appreciably assist in heat removal to the containment walls.